This invention relates to a thyristor control methode in a thyristor AC-DC converter for converting an AC power into a DC power by a six-phase, half-wave or three-phase, full-wave back-to-back or anti-parallel connection thyristor bridge.
For instance, a DC servo type control device requires, for its starting, a supply of a large current to its DC motor because of mechanical inertia but, in many cases, it is necessary to supply a positive or negative small current to the DC motor after a rapid acceleration or deceleration. Where a thyristor AC-DC converter is employed in such a DC servo type control device, a three-phase, half-wave back-to-back connection bridge is adopted recently but a supply of a large current of large ripples inflicts serious damage on the motor commutator. For supplying a current of as small ripples as possible, it is desirable to use an AC-DC converter of the type employing a six-phase, half-wave or three-phase, full-wave back-to-back connection thyristor bridge. In the DC servo type control device, it is impossible to supply the DC motor with a current corresponding to a small error signal because of the dead zone of the AC-DC converter employing the thyristor bridge. Accordingly, it is the practice in the art to supply the DC motor with positive and negative currents alternately with each other to achieve a control corresponding to the difference between the positive and negative currents. Such a control is possible with the three-phase, half-wave back-to-back connection bridge in the prior art. In the case of the six-phase, half-wave or three-phase, full-wave back-to-back connection, however, when applying positive and negative currents in an attempt to supply them to the DC motor, a current flows to short-circuit the secondary winding of a transformer due to the thyristor and no current flows in the DC motor, so that it is impossible to achieve such a control by alternate application of the positive and negative currents to the DC motor.
One object of this invention is to provide a thyristor control methode for a back-to-back or anti-parallel connection thyristor bridge AC-DC converter with which it is possible to supply a load with a large current of small ripples and to alternately apply positive and negative currents to the load with a small control signal.
Another object of this invention is to provide a thyristor control methode for a back-to-back connection thyristor bridge AC-DC converter which neither impairs the commutator of a DC motor, i.e. a load, nor necessitates the use of a special and expensive commutator nor provides any dead zone and enables an accurate control of the DC motor.